The measurement of Ca++ is fundamentally important in the study of various physiological phenomena in which Ca++ plays a central role. The Ca++ sensitive bioluminescent protein aequorin has been used to detect and monitor Ca++ in various biological systems including living cells, and it has been particularly useful in studying the intracellular distribution of Ca++. Despite its wide use in the past, aequorin has certain shortcomings in the kinetics of its light-emitting reaction as well as in recycling the protein for repeated use. Furthermore, one hitherto unrecognized shortcoming, i.e., the EDTA-binding of aequorin that causes the inhibition of luminescence, has been very recently discovered. The chief objective of this research is to make the aequorin method much more useful by chemical modification of the aequorin molecule. Thus, we plan to study: (1) The effect of acylation, and other means of chemical modification, of aeuorin on the kinetics of the luminescent reaction and on the EDTA-binding affinity of this protein. We ultimately hope to develop a modified form of aequorin that does not bind EDTA and luminesces in direct proportion to the concentration of Ca++. (2) The immobilization of aequorin and its modified forms on gels, glass or Nylon to make practical the recycling and repeated use of this protein. (3) A new technique of measuring Ca2++ utilizing apoaequorin-coelenteramide complex, whose fluorescence is a function of Ca++ concentration.